[ohohyodafarted]

Purple glow between elements of an electron gun is not a good sign. The voltages a crt tester produces are rather low compared to what the crt will be using when it is in operation.

Typically, if you see inter-element purple glo, and you use a Tesla coil to test if the tube is gassy, you will find that the entire gun area in the neck will light up with a gassy pink/purple glo.

I purchased a 15GP22 about a year ago. On my Beltron tester, I was getting very high emission readings, but there was faint inter-element glow in the gun elements. The With a B&K 465 crt tester the glow is much fainter. When I applied the high voltage from my Tesla coil, the entire neck area lit up bright pink/purple showing that the tube was gassy. You can also see a blue-violet glow between the G1 and G2 gun elements

There are 2 ways to be sure if the tube is gassy, Tesla coil or an operational chassis. I have found anything else will not be a definitive test for gas.

Three photos of gassy 15GP22. Beltron emission readings, Inter-element glow on the Beltron crt tester, and Lots of glow with the Tesla coil.

Posting good photos of the kind of glow your 21AX produces would be more helpful.

All that said, there is still a possibility your tube may be OK.